1. Field of the Invention
This invention relates to an antenna, and more particularly to a physically shortened antenna including a conductor having a plurality of folds to permit operation within a limited space.
2. Description of the Prior Art
Antennas generally comprise a conductor or a system of conductors used either for radiating electromagnetic energy into space or for collecting electromagnetic energy from space. Since it is difficult to radiate large amounts of power efficiently in the lower frequency ranges with shortened antennas, it is especially important that as much of the available signal at the transmitter be converted into radiated energy as possible, and that as much of the radiated energy as possible be picked up at the receiver.
A half-wave length conductor is the simplest of the radiating elements. Considerable radiation occurs in this element because of its resonant characteristics and its ability to store large amounts of energy in induction fields. Resonance causes high voltage and high circulating currents and they, in turn, produce strong fields around the antenna. It should be understood that due to resistance of the wire, the movement of waves along an entire wire is somewhat slower than wave movement in space. Wave length on a wire, therefore, is slightly less than that of a wave travelling in space. Physically, an antenna is about 6% shorter than a half-wave travelling in space. This phenomena is sometimes referred to as "end effect".
A quarter-wave grounded antenna is a common type of grounded antenna. This type is called a Marconi antenna as contrasted with a half-wave ungrounded dipole antenna, which is sometimes called a Hertz antenna. The input impedance to the Marconi antenna is approximately 37 ohms when the antenna is fed at its base. In addition, a quarter-wave Marconi is resonant and displays zero reactance as does a half-wave antenna. This assumes that the conducting plane is a perfect conductor. If it is not a perfect conductor, as is the usual case, some of the conditions just discussed would be altered. The conducting plane might be the skin of an aircraft or the frame of an automobile. With ground equipment, the conducting plane is the earth's surface or an artificial ground called a counterpoise.
Energy may be fed to an antenna in a variety of ways. When the exitation energy is introduced to the antenna at the point of high circulating currents, the antenna must be center-fed. When the energy is introduced at the point of maximum voltage, the antenna must be voltage fed.
A folded dipole is a full length conductor that is folded to form a half-wave element. It consists of a pair of half-wave elements connected together at the ends. In it, the voltage at the ends of each element, must be the same. In operation, the field from the driven element induces a current in the second element. This current is the same as the current in the driven element.
Quarter-wave vertical antennas are usable over a narrow band of frequencies around resonance. When it is necessary to modify the electrical length, the electrically short antenna may be lengthened by adding series inductance; the electrically long antenna may be shortened by adding series capacitance. Although the feed impedance or radiation resistance of a quarter-wave length vertical grounded antenna is 36 ohms, higher impedance is available by advancing upwards from ground potential along the length of the antenna. Thus, one point might be a feed point for a 36-ohm line, while a point somewhat higher up along the antenna would serve as a feedpoint for a 52-ohm line. Of course, ground is one of the connections for the transmission line in both instances.
With the conventional antennas utilized heretofore, available space may preclude their use. Vertical grounded antennas have been "shortened" by adding an electrically conductive sphere at the upper end thereof to provide additional capacitance. The use of such a "capacitance ball" does not always provide a satisfactory alternate.
Although in some instances, it is important that antennas, such as radar antennas, have directivity, i.e., an antenna that radiates more energy in one direction than another, other applications require that the antenna be non-directional, isotropic or omni-directional. Accordingly, it is an object of the present invention to provide a new and novel omni-directional antenna.
A further object of the present invention is to provide an antenna, including a plurality of cylindrical antenna sections which are selectively coupled in series circuit relation to provide a variable length antenna having a selected length which is a multiple or sub-multiple of a quarter-wave length at a selected operational frequency.
Still another object of the present invention is to provide an antenna, including a plurality of folded antenna sections which can be selectively coupled to provide a total length of antenna which is a multiple or sub-multiple of a quarter-wave length of the operating frequency.
A still further object of the present invention is to provide an antenna of the type described, including mechanism for mechanically controlling the length of the antenna by means of telescoping antenna sections.
Still another object of the present invention is to provide an antenna, including a plurality of variable length antenna folds which can be tuned to different frequencies by changing the physical lengths of selected folds.
A further object of the present invention is to provide an antenna, including a plurality of coaxial, radially spaced, axially extending, current conducting, different length cylinders which are selectively coupled together.
A still further object of the present invention is to provide an antenna, including a plurality of radially spaced antenna elements each including a pair of radially spaced antenna segments each of which includes telescoping cylinders which can be adjusted to tune the antenna to different wavelengths.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.